1. Field of the Invention
The invention relates in general to a method of removing a silicon-on-glass residue, and more particularly, to a method of preventing the contact window from being poisoned by the residue of silicon-on-glass.
2. Description of the Related Art
Contacts have been widely applied as the multi-level interconnects between integrated circuits or semiconductor devices. In a conventional method for forming a contact, a inter-metal dielectric layer (IMD) is formed on a first wiring layer. The inter-metal dielectric layer has an opening therewithin to expose a part of the first wiring layer. A conductive layer is formed to fill the opening as a contact plug as for the interconnection between the first wiring layer and a second wiring layer formed thereafter.
Typically, the inter-metal dielectric layer comprises layers of spin-on-glass layers. Spin-on-glass is a common planarization technique to form a relative flat layer. The process comprises dissolving dielectric material into a solvent, and using spin-coating technique to cover the material on a wafer. Since the dielectric material is circulated on the wafer with the solvent, the uneven recessed surface of the wafer are easily filled with the dielectric to obtain a planarized surface. After thermal treatment, the solvent is removed to cure the dielectric material as a spin-on-glass layer. A local planarization is thus achieved. Therefore, the technique of spin-on-glass is advantageous to gap filling for preventing the formation of a void while depositing a dielectric layer.
The sandwich type spin-on-glass layer is widely applied in semiconductor process. FIG. 1A to FIG. 1F are cross sectional views showing a conventional method for forming the interconnect with a sandwich type spin-on-glass layer as an inter-metal dielectric layer. In FIG. 1A, a semiconductor substrate 100 comprising metal layers 114, 116, and 118 is provided. A top view of the substrate 100 is shown as FIG. 2A, and FIG. 1A is taken from the cross section of line I-I'.
In FIG. 1B, a dielectric layer 120 is formed to cover the whole substrate 100, including the metal layers 114, 116 and 118. A spin-on-glass layer 122 is formed on the dielectric layer 120.
In FIG. 1C, the spin-on-glass layer 122 is etched back. Ideally, after the etch back process, the spin-on glass 122 does not only remain in and fill the recessed regions of the surface of the substrate 100 to achieve a local planarization. However, it is often to find that the spin-on-glass layer 122 does not only fill the recessed regions on the surface of the substrate 100, but the spin-on-glass layer 122 also remains on a flat surface region such as the surface of dielectric layer 122 over the metal layer 114. As shown the figure, the spin-on-glass layer remaining on the dielectric layer 122 over the metal layer 114 is denoted as 120a, whereas the spin-on-glass layer filling the recessed regions is denoted as 122a.
In FIG. 1D, a dielectric layer 126 is further formed to cover the dielectric layer 120 and the remaining spin-on-glass layer 120a and 122a. The dielectric layer 120, the spin-on-glass layer 122a and 120a, and the dielectric layer 126 compose a sandwich type spin-on-glass layer.
In FIG. 1E, to achieve the interconnection between any of the metal layers 114, 116, 118 and a second wiring layer formed thereafter, the sandwich type layer is patterned to form openings 128 and 130 by photolithography and etching process. The openings 128 and 130 penetrate through the sandwich type spin-on-glass layer to expose the metal layer 114 and 118, respectively. As shown in the figure, a part of the remaining spin-on glass layer 120a is exposed on the side walls of the openings 128 which expose the metal layer 114. As mentioned above, the spin-on-glass layer 120a is formed from curing the solution containing dielectric material. Therefore, it is very often that the solvent is not removed completely during curing process. Or during the photolithography and etching process for patterning the openings 128 and 130, the remaining spin-on-glass layer 120a is very easy to absorb moisture. For either the remaining spin-on-glass layer 120a to containing solvent or moisture, the contained solvent or moisture evaporates, namely, the spin-on-glass layer 120a outgases, in a higher temperature. A top view of this figure is shown as FIG. 2B.
In FIG. 1F, a metal layer 137 is formed to fill the openings 128 and 130 to form contact plugs. The temperature for forming the metal layer 137 is typically so high to cause the solvent or moisture contained in the spin-on-glass layer 120a to evaporate, that is, the spin-on-glass layer 120a to outgas. The metal layer 137 is thus filled with gas void 132 evaporated from the solvent or the moisture absorbed by the spin-on-glass layer 120a. The metal layer 137 is thus poisoned. Since the metal layer 137 is formed to electrically connect the first wiring layer 126 and a second wiring layer formed thereafter, with the formation of the gas voids 132, a poor conductivity is obtained. Therefore, the quality of the device is degraded.
On the other hand, the remaining spin-on-glass layer 120a over the metal layer 114 causes the aspect ratio of forming a contact plug to increase. The higher the aspect ratio is, the poorer the step coverage is while depositing the metal layer 137. As a result, an air void 134 as shown in the figure is formed.